Generally, as illustrated in FIG. 8, a wireless communication system includes mobile terminals 10, radio base stations 30, a mobile communication core network 50 and a network monitoring system 90.
The mobile terminal 10 transmits and receives data such as communication traffic or control traffic to and from the radio base station 30.
The radio base station 30 transmits and receives data to and from the mobile terminals 10 and the mobile communication core network 50 connected through wired links 40. In addition, around the radio base station 30, a radio cell 35 is provided as a range in which wireless links 20 to the mobile terminals 10 are effective.
The radio base station 30 manages the radio cell 35 provided therearound. The mobile terminals 10 exist in the radio cell 35 managed by the radio base station 30, and transmit and receive data to and from the radio base station 30.
The mobile communication core network 50 is realized by a switching point or a server device which is not illustrated. The mobile communication core network 50 is connected with the radio base stations 30 through the wired links 40, is also connected with an external network 70 through a wired link 60, and transmits and receives data to and from the radio base stations 30 and the external network 70.
The network monitoring system 90 is connected with the radio base stations 30 through the wired links 80, is connected with the mobile communication core network 50 through the wired link 81, and monitors quality, monitors failure and manages a configuration of the wireless communication system.
In such a wireless communication system, radio parameters of the radio base stations 30 need to be adequately set to satisfy predetermined communication quality in an entire service area of a service target. The radio parameters are typically, for example, total transmission power of the radio base stations 30, tilting angles of radio base station antennas in vertical and horizontal planes, a neighbor cell list and a handover threshold. Hereafter an explanation is made with assuming that a neighbor cell list is set to the radio base station 30 as a radio parameter.
Hereinafter, the neighbor cell list will be described. In the wireless communication system, services are provided by two dimensionally arranging radio cells in the entire service area wholly. When moving from a radio cell of a connection destination to another radio cell, the mobile terminal 10 performs switching processing of a connection destination radio cell which is referred to as “handover”. Although an expression that a mobile terminal connects to a radio cell will be presented below, more specifically, this means that a mobile terminal connects to a radio base station which manages a radio cell. Further, handover processing is, more specifically, directed to switching a connection destination of a mobile terminal to a radio base station which manages another radio cell.
This handover processing will be realized as follows. The mobile terminal 10 is set (also referred to as “instructed” below) in advance to measure radio quality of a radio cell of a connection destination and neighbor cells neighboring to this radio cell, and, when radio quality of the connection destination radio cell deteriorates, transmits (also referred to as “reports” below) measured radio quality of the radio cell and the neighbor cells to the radio base station 30 which manages the connection destination radio cell. Further, at a point of time when receiving a report from the mobile terminal 10, the radio base station 30 decides a handover destination radio cell to realize handover.
In this case, a method of restricting handover destination radio cell candidates to a specific cell among neighbor cells is generally used to reduce a load on the mobile terminal 10 and process handover at a high speed. Meanwhile, the handover destination radio cell candidates are indicated by a neighbor cell list. The neighbor cell list is generated when communication carriers register radio cells per radio cell, and is transmitted (also referred to as “broadcasted” below) to the mobile terminals 10 from the radio base station 30 which manages the radio cell in downlink. Radio cells of neighbor cells which are registered in the neighbor cell list are referred to as “listed cells”, and radio cells which are not registered in the neighbor cell list are referred to as “detected cells”.
Hereinafter, an example will be described where formula (1) disclosed in Non Patent Literature 1 is used as operation conditions when the mobile terminal 10 reports measurement information to the radio base station 30.Ps+Os<Pt+Ot  Formula (1)
Ps and Pt represent received powers of pilot signals transmitted from the radio base station 30 which manages a connection destination radio cell and the radio base stations 30 (also referred to as “neighbor base stations 30” below) which manage neighbor cells. Os and Ot represent offset values of the received powers. Os works on the received power of the pilot signal transmitted from the radio base station 30 which manages the connection destination radio cell. Further, Ot works on the received power of the pilot signal transmitted from the neighbor base station 30. In addition, a value which varies per neighbor cell can be set to Ot.
When the operation conditions of formula (1) are set to the radio base station 30, this radio base station 30 transmits (also referred to as “notifies” below) the operation conditions of formula (1) to the mobile terminals 10 which connect to the managed radio cell. When the received powers of the pilot signals transmitted from the radio base station 30 which manages the connection destination radio cell and the neighbor base stations 30 satisfy the conditions of formula (1), the mobile terminal 10 is triggered to report measurement information of the pilot signals transmitted from the neighbor base station 30 to the radio base station 30.
In addition, when the conditions of formula (1) are satisfied, the mobile terminal 10 is triggered to report measurement information to the radio base station 30, and therefore formula (1) is referred to as “trigger conditions” in sometimes. Further, the parameter ThHO is also referred to as a “handover decision threshold” (handover threshold) by rewriting formula (1) to formula (2). This is because formula (1) is used as the trigger conditions for reporting deterioration of communication quality of a connection destination radio cell upon handover execution decision, from the mobile terminal 10 to the radio base station 30.Pt−Ps>ThHO ThHO=Os−Ot  Formula (2)
According to such operation conditions, the radio base station 30 which receives a report of measurement information from the mobile terminal 10 generally decides a handover destination radio cell from listed cells of which radio quality is reported. Hence, the mobile terminal 10 cannot be generally handed over to a detected cell. Therefore, when there are radio cells registration of which is omitted in the neighbor cell list, there are concerns that a mobile terminal cannot be handed over to an adequate cell and then abnormal disconnection occurs halfway, and the mobile terminal is handed over to an inadequate radio cell and then communication quality deteriorates. Hence, how a neighbor cell list without omission of registration is generated is important to secure good communication quality.
Meanwhile, to suppress a load of a network which is required to broadcast the neighbor cell list and a load of the mobile terminal 10 which is required to measure and report radio quality, the number of radio cells registered in the neighbor cell list generally has an upper limit (LMax). Hence, it is necessary to preferentially register a radio cell which is highly likely to contribute to improving communication quality, in the neighbor cell list such that main radio cells are registered in the neighbor cell list.
Generally, a neighbor cell list requires an advanced technique for setting, and is optimized based on a result of a driving test in a service area of the wireless communication system. However, in recent years, a neighbor cell list optimizing system is studied which autonomously optimizes neighbor cell list while a wireless communication system is operated.
FIG. 9 is a view illustrating an example of a configuration of a wireless communication system which has the neighbor cell list optimizing system. A wireless communication system illustrated in FIG. 9 includes the mobile terminals 10, the radio base stations 30, the mobile communication core network 50, the network monitoring system 90 and a neighbor cell list optimizing system 95. In addition, in FIG. 9, the same configurations as in FIG. 8 will be assigned the same reference numerals, and will not be described.
The neighbor cell list optimizing system 95 is connected with the network monitoring system 90 through a wired link 82, and optimizes the neighbor cell list for the radio cell 35 managed by the radio base station 30.
Non Patent Literature 2 and Patent Literature 1 disclose neighbor cell list updating methods of neighbor cell list optimizing systems.
FIG. 10 is an explanation chart illustrating a neighbor cell list updating method disclosed in Non Patent Literature 2. In addition, according to the method disclosed in Non Patent Literature 2, a mobile terminal measures received power of a pilot signal transmitted from a radio base station as radio quality, and reports a connection destination radio cell to the radio base station.
According to the neighbor cell list updating method disclosed in Non Patent Literature 2, the number of trials (a) of handover to a listed cell is counted per listed cell registered in the neighbor cell list. Further, the number of reports (n) from mobile terminals that received power of a pilot signal transmitted from a radio base station which manages a detected cell is a threshold or more is counted per detected cell. Furthermore, detected cells the number of reports (n) of which is a threshold (ThAdd) or more are registered in the neighbor cell list in order from the radio cell of the highest number of reports (n), and listed cells the number of trials (a) of which is a threshold (ThDel) or less are deleted from the neighbor cell list to update the neighbor cell list.
FIG. 11 is an explanation chart illustrating a neighbor cell list updating method disclosed in, for example, Patent Literature 1. In the wireless communication system disclosed in Patent Literature 1, a measuring radio device 5A is arranged in a radio cell (new cell) 35A managed by a radio base station 30A which does not yet provide communication service after the radio base station 30A is newly installed. Further, a mobile terminal 10B is connected to a radio cell (existing cell) 35B managed by a radio base station 30B which has already provided communication service.
According to the neighbor cell list updating method disclosed in Patent Literature 1, the measuring radio device 5A measures a signal intensity of a pilot signal transmitted from a radio base station which manages an existing cell, and reports the signal intensity to the radio base station 30A. The radio base station 30A requests the radio base station 30B which transmits a pilot signal of which measurement value of the signal intensity is a threshold or more to measure in the mobile terminal 10B connected to the existing cell 35B the signal intensity from the radio base station 30A and report the signal intensity to the radio base station 30A. Further, the radio base station 30A registers in the neighbor cell list the existing cell 35B to which the mobile terminal 10B which measures the signal intensity equal to or more than a threshold is connected, and updates the neighbor cell list of the new cell 35A.
Furthermore, Patent Literature 2 discloses a method which is directed to reducing a measurement report stand-by time of neighbor cells as a related technique. Patent Literature 3 discloses a method which is directed to maintaining precision of line quality required by a base station to allocate the frequency.